As used herein the term "ophthalmic lens" means vision correction lenses such as contact lenses and intraocular lenses. Other, less common, vision correction lenses such as artificial corneas and intralamellar implants are also included in this definition.
Bifocal spectacle lenses have been known for hundreds of years. In such lenses a first region of the lens is typically provided with a first focal length while a second region of the lens is provided with a second focal length. The user looks through the appropriate portion of the lens for viewing near or far objects.
More recently there has been interest in developing other types of multifocal ophthalmic lenses. Multi-focal contact lenses utilizing an approach similar to that used in spectacle lenses are described in Contact Lenses: A Textbook for Practitioner and Student, Second Edition, Volume 2 on pages 571 through 591. Such lenses have serious drawbacks, however, because they require that the lens shift on the eye so that different portions of the lens cover the pupil for distant and close vision. This design cannot be used for intraocular lenses or other implanted lenses, because such lenses cannot shift. Even for contact lenses the design is disadvantageous because it is difficult to insure that the lens will shift properly on the eye for the desired range of vision.
In another design for a bifocal contact lens described in the above-referenced textbook, a central zone of the lens is provided with a first focal length and the region surrounding the central zone is provided with a second focal length. This design eliminates the necessity for shifting the lens by utilizing the phenomenon of simultaneous vision. Simultaneous vision makes use of the fact that the light passing through the central zone will form an image at a first distance from the lens and light passing through the outer zone will form an image at a second distance from the lens. Only one of these image locations will fall on the retina and produce a properly focused image while the other image location will be either in front of or behind the retina. The human eye and brain will, to a great extent, work together to ignore the improperly focused image. Thus the user of such a lens receives the subjective impression of a single well-focused image. A disadvantage of such a lens is that, if the central zone is made large enough to provide sufficient illumination in its associated image in low light situations, i.e. when the patient's pupil is dilated, the central zone will occupy all or most of the pupil area when the pupil contracts in a bright light situation. Thus bifocal operation is lost in bright light. Conversely if the central zone is made small enough to provide bifocal operation in bright light situations, an inadequate amount of the light will be directed to the image associated with the central zone in low light environments. Because the central zone is commonly used to provide distant vision, this can create a dangerous situation when the user of such a lens requires distant vision in low light situations such as when the user must drive a motor vehicle at night.
U.S. Pat. Nos. 4,210,391; 4,340,283; and 4,338,005, all issued to Cohen, teach the use of a plurality of annular regions that direct light to multiple foci and rely upon simultaneous vision to discard unfocused images. They teach the use of alternating concentric Fresnel zones, wherein each of those zones have substantially equal area. The use of such equal area zones causes the lens to provide a diffractive focus of the light. A first focus will occur for the zero order diffracted light while a second focus will occur for the first order diffracted light. Such a structure is known as a diffractive zone plate.
A diffractive zone plate must be designed for light of a particular wavelength and will work most efficiently for light at that wavelength. The radius of the n.sup.th zone (r.sub.n) in the diffractive zone plates taught in the Cohen patents will be equal to .sqroot.n r.sub.1 where r.sub.1 is the radius of the central zone. To a reasonable approximation r.sub.1 would be equal to .sqroot..lambda.f where .lambda. is the design wavelength and f is the focal length of the diffractive structure. Therefore the n.sup.th zone would have a radius equal to .sqroot.n.lambda.f.
In designing a diffractive zone plate a design wavelength must be selected. When a desired focal length and wavelength are selected for a lens as taught in the Cohen patents, the area of each of the zones, and thus the location of the boundary of each zone, are determined. This rigid definition of the zones result in a disadvantage to the zone plate structure. In order to obtain an efficient diffractive bifocal operation, a sufficient number of zones must be used. However if the area of the central zone is too large, under bright light situations with the pupil constricted, only a single zone or very few zones will be utilized. Thus the efficiency of the multi-focal operation is greatly reduced.